The present disclosure relates to an image forming apparatus provided with a developing device for developing an electrostatic latent image formed by electrophotography.
A developing device employing a so-called touch-down type (also referred to as a hybrid type) is known. In an apparatus employing this type, a two-component developer including at least a carrier and toner is supported on a surface of a developer supporting rotator to form a magnetic brush on the surface by the carrier. The toner supplied from the magnetic brush to a toner supporting rotator forms a toner layer on a surface of the toner supporting rotator, such that the toner flies from the toner layer to an image carrier. In this manner, an electrostatic latent image on a surface of the image carrier is developed into a toner image.
In the developing device of the touch-down type, a so-called development history may easily occur, which is a difference in developing performance at a time of subsequent rotation of the toner supporting rotator between two parts of the toner supporting rotator. One part is where toner is used by development and another part is where unused toner remains on the surface of the toner supporting rotator, of the toner of the toner layer formed on the surface of the toner supporting rotator. Such a development history results in unevenness of the toner layer formed on the surface of the toner supporting rotator at a time of the subsequent rotation. As a result, it my be that a reduction in density of an image occurs due to a change in toner characteristics (particle size, triboelectric charge and the like), degraded image quality occurs due to long-neglected undeveloped toner, and scattering of the toner and developing defect such as developing ghost occurs.
In order to suppress an occurrence of the abovementioned developing defect, stripping of the undeveloped toner layer on the surface of the toner supporting rotator and formation of a new toner layer on the surface of the toner supporting rotator are performed between two successive operations of development and at a time without transferring. In this case, it is possible to improve the developing ghost if the stripping performance of the undeveloped toner layer (increasing stripping of the undeveloped toner) is prioritized. However, it is likely that the image density may decrease due to insufficient formation of the new toner layer. On the other hand, if formation of a new toner layer (increasing thickness of the toner layer) is prioritized, the reduced image density will not occur. However, the developing ghost may worsen.
In order to satisfy the need of solving the problems of developing ghost and reduced image density, it is performed to set various values to an AC/DC superposed bias voltage that is applied to the developer supporting rotator and the toner supporting rotator. However, since the magnitude of DC component has a great impact on a thickness of the toner layer, it influences density and quality of an image. In addition, if an AC voltage component is increased, stripping performance of the undeveloped toner layer from the toner supporting rotator increases and the developing ghost is improved. However, the developer may easily stay in a nip portion. Accordingly, it is likely that the blockage caused by the developer occurs at the nip portion and the leakage due to an excessive bias voltage occurs.
In order to avoid abovementioned side effects, the bias voltage must be set to an optimal value. However, the optimal bias voltage value depends greatly on an amount of charged electricity and the like of the developer, and the amount of charged electricity and the like of the developer depend greatly on humidity, temperature and the like. In other words, the optimal bias voltage value depends on environmental conditions under which the image forming apparatus is placed.
Furthermore, the toner layer stripping performance of the developing device of the touch-down type is also greatly influenced by a size of a gap between the developer supporting rotator and the toner supporting rotator. Since the gap depends on the manufacturing accuracy, assembly accuracy and the like of components, the gap varies from one image forming apparatus to another. Accordingly, the optimal bias voltage value varies from one image forming apparatus to another.
A developing device is known, in which two magnetic rollers are configured for dedicated functions of toner layer formation and toner layer stripping, respectively, in order to solve the abovementioned problems. More specifically, the two magnetic rollers are arranged opposite to the toner supporting rotator. One of the two magnetic rollers is dedicated to a developer supporting function for forming a toner layer on the toner supporting rotator and the other is dedicated to a stripping function of an undeveloped toner layer on the surface of the toner supporting rotator.
However, if the two magnetic rollers are installed inside the main body of the abovementioned developing device, it is necessary to apply bias voltages of different values to the two magnetic rollers. This makes the mechanism of the developing device complex, and increases the size of the entire image forming apparatus for securing a space for accommodating large magnetic rollers inside the developing device.